1. Field of the Invention
The present invention relates to a process for producing gallium arsenide wafers having few crystal defects and a uniform low dislocation density and a (100) plane or a plane inclined within a few degrees of the (100) plane.
2. Description of the Prior Art
Because of its unique nature, gallium arsenide is currently being used in semiconductor devices as a substrate crystal for epitaxial growth or ion implantation. In most cases, the gallium arsenide substrate crystal used as a wafer is sliced from a rod-shaped single-crystal boule produced by a "boat" method such as the horizontal Bridgman method, a gradient freezing method, or a pulling method. Conventionally, there are two requirements that must be met by these wafers. One is that they are round or circular because the circular form is easy to obtain by jigs in the process of epitaxial growth. Also, this form helps standardize the manufacturing requirements of semiconductor devices, facilitates the automatic work of wafer processing and has a high reproducibility. The second requirement is that the wafers have few crystal defects and uniform low dislocation density so as to extend the life of semiconductor devices and improve their performance.
The nature of the boat method is such that it is not capable of producing a round bar of gallium arsenide single-crystal boule but instead provides a boule 1 in the form of a longitudinal half of a cylinder as shown in FIG. 1. Such a boule often presents much difficulty in handling the triangular-like wafers including so-called "D-shape" wafers that are sliced from it in a direction oblique to the direction of crystal growth. The conventional alternative is to slice circular wafers from a rod-shaped gallium arsenide single-crystal boule 2 (FIG. 2) produced by the pulling method. But as shown by the hatched area in FIG. 3, a circular wafer 3 produced by the pulling method has a high density of dislocations as well as fairly great number of crystal defects throughout.
With the increase in the demand for gallium arsenide semiconductor devices of improved performance, the manufacture of gallium arsenide single-crystal boules from which circular wafers having few crystal defects and a uniform low dislocation density can be cut is especially desirable.